Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?

Annotations already declare readOnlyHint, idempotentHint, and destructiveHint, and the description adds meaningful behavioral context: the 100-IBAN cap, cost per IBAN, free-tier allowances, return shape, and an API key generation endpoint. Nothing in the description contradicts the annotations.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Is the description appropriately sized, front-loaded, and free of redundancy?

The description is front-loaded with the main purpose and uses clear labeled sections for usage, returns, and cost. Cost information appears in both the opening sentence and the COST section, creating some redundancy, but each section still contributes unique guidance such as free-tier details and the API key endpoint.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

Given the single-parameter schema, existing output schema, and safety annotations, the description covers everything needed to select and call the tool correctly: batch limits, return shape, when to use it, cost, and alternative routing. There are no material gaps.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

The schema already documents the only parameter, ibans, as an array of 1-100 strings with 100% coverage. The description adds behavioral context around batching and cost but does not need to explain the parameter format further, so the schema-does-the-heavy-lifting baseline of 3 applies.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Does the description clearly state what the tool does and how it differs from similar tools?

The description opens with 'Validate up to 100 IBANs in a single call', which identifies the exact verb, resource, and batch scope. It also clearly differentiates from validate_iban by emphasizing the 100-IBAN batch capability and the per-IBAN cost comparison.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Does the description explain when to use this tool, when not to, or what alternatives exist?

The description provides an explicit 'USE WHEN' section listing concrete scenarios and an explicit decision rule: 'whenever you would otherwise call validate_iban more than 2-3 times in a row.' It names the alternative tool directly, making routing between tools unambiguous.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.